Process of coating a silicon semiconductor with indium using an ion beam



L. FEDOWS-FEDOTOWSKY PROCESS OF COATING A SILICON SEMICOND Dec. 27, 19663,294,583

UCTOR WITH INDIUM USING AN ION BEAM IN VEN TOR F EDOTOWSKY Chi/M66; amd244%,

Filed June 14, 1962 HIS ATTORNEYS HROCESS F (IDATING A SZLICGNSEMECGNDUC- TOR WETH lNDlUM USING AN IUN BEAM Leonid Fedows-Fedotowsky,North Adams, Mass, assignor to Sprague Electric Company, North Adams,Mass,

- p a corporation of Massachusetts Filed June 14, 1962, Ser- No.2ii2,5t)4 1 Claim. (Cl. 117-227) This invention relates to the formationof contacts or electrodes and more particularly to the production ofcontacts or electrodes on semiconductor devices.

The bringing together of the contact material and a semiconductor is acritically sensitive operation. It is complicated by the designrequirements of the product, the limitations of space caused by theminuteness of the devices. the characteristics of the materials used andthe desirability of automatic operation.

The common prior art techniques for applying contacts to semiconductorsinclude evaporative plating, chemical platin and electroplating'usinglocal contacts. Evaporative plating requires cumbersome equipmentutilizing interposed masks between the source and the surface to becontacted. This technique also is limited to materials havingcomparatively low vapor pressures, such as indium and aluminum.Materials such as phosphorus, arsenic and antimony have vapor pressurestoo high for this method. 'Chemi-plating has proved to be unsatisfactoryin that the layers produced have typically been too thin.Electroplating, using local contacts, produces uneven plating becausethe local contacts, when coupled with the relatively poor conductivityof the semiconductor material, results in a non-uniform current densityacross the surface to be plated. Furthermore, whenever liquids are usedduring contact formation the likelihood of contamination is greatest.

It is an object of the present invention to overcome the foregoing andrelated problems.

Another object is to present a method for forming rectifying or ohmiccontacts on a semiconductor.

Still another object is the formation of contacts of controlled shapeand thickness.

Yet another object is the formation of contacts in the absence ofmechanical pressure on the semiconductor.

A further object of the instant invention is the formationof alloys ofmetals and metalloids on a semiconductor.

' A still further object is the deposit of a dielectric on asemiconductor.

Another object is the deposit of a contact on a semiconductor withoutthe aid of interposed masks.

Other objects and advantages of the present invention will be madeobvious to those skilled in the art by the following description whenconsidered in relation to the accompanying drawing, in which:

The sole figure is a sectional view of an apparatus employed in theprocess of this invention.

In general, this invention involves the ionization of a material in anion source, (indium trimethyl, In(CH will be used for purposes ofillustration), extraction of the ions from the ion source, accelerationof the ions, shape definition of the ion beam, separation of thenon-metallic ions from the indium ions, correction of sphericalaberration, deceleration of the ions and the simultaneous discharge ofthe indium ions and deposit of indium metal on the semiconductor targetand collection of the separated lighter ions.

The overall aspects of this invention will be best understood byreference to the drawing which shows an embodiment of the apparatusemployed to illustrate the process of this invention. This apparatuscomprises a United States Patent 0 3,294,583 Patented Dec. 27, 1966 mainchamber it in which the ions are produced, extracted, accelerated,defined, focused, separated and decelerated and a target chamber 11 inwhich the ions are discharged, the metal deposited and the lighter ionscollected. The two chambers are maintained during operation under avacuum of between 10- and 10 mm. Hg by means of vacuum pumps 20. Thechambers are separated by a lock 21. This lock permits sealing of themain chamber it while the semiconductor target 22 is being replaced witha new target in the target chamber 11.

The main chamber 10 is made up of any suitable ion source 12, an ionextracting electrode 13, a shape defining electrode 14, an einzel lens15, a magnetic deflector 16, an electric prism deflector 17 and an iondecelerator 18. The target chamber 11 comprises a chamber lock 21, asemiconductor target 22, a target holder 23, a microscope 24, a viewingwindow 25, a mirror 26 and a light ion or proton collector 27. Thevarious components of the two chambers are energized by appropriatevoltages transmitted through lead-wires 19.

In the interest of clarity, einzel lens 15 and the combination ofmagnetic deflector i6 and electric prism deflector 17 have been shown asseparate components. It is to be understood, however, that in actualpractice 16 and 17 would be combined with the third electrode of theeinzel lens in order to conserve space.

A preferred ion source employed in the instant process is acapillary-arc ion source. This device comprises a tungsten cathode andan anode, each positioned in di ametrically opposed chambers. Thechambers are connected by a capillary restriction, within which theionization of gas molecules takes place, see A Handbook on MassSpectroscopy, by Mark G. Inghram et al., Nuclear Science Series ReportNo. 14, National Academy of SciencesNational Research Council,Washington, DC. 1954, pages 29 and 30. The particular type of ion source12 contemplated herein is not critical. Considerations of operationalconvenience generally will dictate which of the manyliterature-described ion sources can best be adapted to the presentprocess, see e.g. Inghram et al., supra, pages 32 and 33 (FIG. 22) andModern Mass Spectrometry, G. P. Barnard, The Institute of Physics,London 1%3, pages 56-58 (FIG. 23). Other types of ion sources employinghigh temperatures in evaporating crucibles, or high voltages indischarge chambers, or gas discharges in radio frequency fields, etc.may also be used.

Example Using indium as the metal to be deposited on the semiconductortarget and using the above-described capillary discharge ion source theprocess is as follows: The metal is introduced into ion source 12 as acompound of the metal in vapor form. Indium trimethyl, which sublimes atroom temperature, is the indium compound employed. The cathode rayionizes the molecules into In+ ions and charged fragments of CH e.g. C-Hfragments and I-I+ ions. For convenience the fragments of CH will bereferred to as protons since a large proportion are H+ ions. An ionextracting electrode 13 removes the ions from the capillary restrictionof the ion source and directs the ions to the shape-defining electrode14. The apertures of the shape-defining electrode and the ion extractingelectrode are in alignment. The instant ion-optical system willfaithfully reproduce on the target but in reduced scale the shape of theaperture in the ion beam shape-defining electrode. The shape of theelectrode desired will determine the shape of the aperture, e.g.circular, square, rectangular and any of countless odd shapes. The shapeof the aperture in 14 of the drawing is square.

After passing through the shape-defining electrode the ion beam isfocused by einzel lens 15. The design of the focusing lens isfacilitated by the fact that the trajectories of the focused ions do notdepend on the specific charge 6/111 of the ions, and hence a lensdesigned for focusing electron rays can be successfully used forfocusing various ions. A size reduction in the order of up to about 10times is required and this can be accomplished with a single lens. Athree tube einzel lens (i.e., a single lens) has been selected as thepreferred focusing lens 15. Lenses of this type are described byKlemperer in Elec tron Optics, University Press, Cambridge 1953, in hisdiscussion of saddle-field lenses. The voltage V applied to the innertube of the lens is equal to 0.2 of the voltage V applied to the outertubes. The voltages applied to both outer tubes are equal. By changingthe voltage V the focal length of the lens can be altered.

Next in sequence the protons are separated from the metal ions. This isaccomplished by the simultaneous action of a magnetic deflector 16 andan electric prism deflector 17. By means of themagnetic field thelighter H+ ion beam is curved away from the target to a greater extentthan the heavier metal ion beam. The resolution of 16 should not be sogreat as to cause separation of metal isotopes, as in mass spectroscopy,but only great enough to cause separation of the protons from the metalions. This is readily accomplished because of the great difference intheir respective masses. The magnetic deflector consists of tworelatively small magnets positioned about a non-magnetic stainless-steeltube, which tube is part of electric prism deflector 17 in the instantillustration.

Since the magnetic field will also act upon the heavier metal ions,though not to the same extent as on the protons, this action must becompensated for in order to reduce spherical aberration. Compensation isbrought about by the action of the electric prism deflector 17 whichdeflects the metal ion beam so that the metal ions strike the target atthe same point they would have struck had the magnetic field not actedupon them.

In order to form an adherent metal deposit on the target it is necessaryto decelerate the indium ions. The indium ions acquire an energy ofseveral thousand electron volts during their passage from the ion source12 through the magnetic deflector 16 and the electric prism deflector17. This corresponds to velocities in the order of 10 cm./sec. At suchahigh velocity an adherent deposit will not be formed on the target. Theparticles will strike with such force as to rebound, taking away smallpieces of the target surface in the process. To overcome this diflicultythe ions are slowed down to thermal velocities of a few electron volts.This is accomplished by interposing tubes or rings 18 between thedeflectors and the target. The rings have gradually diminishingpotentials, with that of the last ring being close to the potential ofthe ion source 12. Ions passing through ion decelerator 18 are sloweddown to a point where they will strike the target, discharge and adherethereto. The deposition can be observed by means of micro-scope 24. Thetarget 22 is connected by lead-wire 29 to an apparatus (not shown) forreading the ion current. Proton collector 27 is likewise connected tosuch an apparatus via lead-wire 29.

By the foregoing example an indium dot was formed having the dimensions4 x 4 x 2 mils. This equals a volume of X A. The number of atoms in thedot are 1.7 x 10 The ions will discharge Q=N.e coulombs which equal 2.7X 10 coulombs. The dot was formed in 10 seconds with a beam current of270 microamperes, using a volume of 0.64 mrn. of indium trimethyl.

In addition to metals, metalloids such as phosphorus, arsenic, antimony,etc. and dielectric elements such as sulfur may also be employed. Thefollowing list of compounds is exemplary of those which may be used inthe present process: A161 SiF PF SP Ni(CO) Ga(CH Ge(CH AsF Cd(CH SnClSbCl Pb(CH3) etc.

The adherence of the electrode deposit to the semiconductor may beinhibited by the presence of an oxide film or some other contaminationon the surface of the semiconductor. These unwanted surface layers canbe removed by directing an electron beam or an ion beam of a neutralgas, e.g. helium, to the electrode area prior to deposition. The lightion or proton beam separated from the metal, metalloid or dielectricelement ion beam may also be directed against the electrode area toclean the surface thereof. This may be accomplished by means of electricprism deflector 17. When using an electron or light ion beam forcleaning purposes the potential of the decelerating electrodes 18 isdecreased in order to take advantage of the high speed of the particles.The electron or ion beam can also be employed to clean the rim orperiphery of an electrode. This produces the same eflect as chemicaletching, widely-known in the art.

Dielectrics can also be deposited on semiconductors. Sulfur, forexample, can be deposited on the semiconductor surface in order tosulfurize the surface. It is an excellent insulator, having aresistivity of 10 ohm-cm. Sulfur can be used in forming a space fillingmaterial over which interconnecting electrodes can be deposited asbridging conductors. The dielectric is deposited between two or moreelectrodes or contacts and then one or more strips or bridges are formedover the dielectric. In the case of sulfur it can be vaporized later,since its low melting point makes it unusable for semiconducting devicesas a permanent insulating material.

Alloys can be deposited by introducing a mixture of I two or more gasesinto the ion source. In this modification a double magnetic deflectionsystem is employed. The first magnetic deflector separates the lighterions or pro tons from the two or more metal ion beams. The secondmagnetic deflector collects the metal ion beams on the target into onepoint or image. When employing this modification it is preferred toemploy the electric prism deflector in conjunction with the two magneticdeflectors because, it is then possible to direct the mixed metal ionbeam to any given location on the target. This mobility would besacrificed by eliminating the electric prism deflector.

The contacts formed by the present process may be rectifying or ohmicdepending upon the metal deposited and the character of thesemiconductor. Alloys of a metal and a metalloid can be formed on thesemiconductor by the successive deposition of the metal first, e.g.lead, and then over this deposit a deposit, slightly smaller in area, ofa metalloid, e.g. arsenic. This is then followed by a heating step toalloy the two layers. In this modification either a single or a multipleion source may be employed.

In forming contacts which are an alloy of two or more metals or of ametal and a metalloid, a heating unit can be employed within chamber 11so that alloying can take place during or after deposition. In someinstances the heat produced on the target due to the kinetice energy ofthe discharging ions will be high enough to cause alloying. It is, ofcourse, also contemplated to alloy outside of the chamber.

By using an assembly such as that shown in chamber 10 on both sides of asemiconductor, it is possible to form transistor type devices. Suchdevices can also be formed by using a single assembly, as in thedrawing, and after deposition on one side of the semiconductor target,the target can be turned or flipped over for deposition on the otherside.

The present process is particularly effective in forming a good contactbetween a comparatively high melting metal and a semiconductor. Normallythere is a considerable thermal expansion coeflicient differentialbetween such materials which causes cracks to form in the contact. Twosuch materials are aluminum and silicon. In forming a strain-freecontact between such material, aluminum is deposited on a silicon wafer,as in the example above, except that a heater means is employed to heatthe silicon. This causes a surface alloying of the aluminum and thesilicon. After cooling, a second layer of aluminum is deposited over thefirst, with heating to cause a blending of the two aluminum layers.Thereafter a small dot of tin or lead is deposited on the aluminum and alead-wire soldered thereto. An electrode formed in this manner will notexhibit a tendency to crack because of a difference in thermalexpansion.

While the foregoing description has been made with reference to theformation of contacts or electrodes on semiconductors, it is obviousthat the process has utility in any instance where a thin film of therecited elements or alloys is required upon a solid substrate. It willbe evident to those skilled in the art that many variations are possiblewithin the spirit of the invention. There is no intention to limit theinvention except as defined by the following claim.

What is claimed is:

A process for treating in a partial vacuum the surface of a siliconsemiconductor body comprising:

(a) ionizing in an ion source an indium compound comprising indium andat least one other ion having a mass less than indium, in combined formtherewith, whereby to form ions of indium and said lighter ions; 1

(b) drawing a beam of the ions out of the ion source by means of an ionextracting electrode;

(c) subsequently directing said beam through a shapedefining electrodeso that said beam will assume the shape of the aperture thereof;

((1) subsequently passing the ion beam through a focusing lens designedto effect a size reduction in the crosssection of said beam of up toabout ten times;

(e) subsequently subjecting said ion beam to the action of a magneticdeflector to separate the lighter ions from the heavier ions at anenergy of several thousand electron volts and a velocity of the order ofcrn./sec.;

(f) subsequently subjecting the separated ion beams to the action of anelectric prism deflector to compensate for the magnetic deflection 0fthe heavier ion beam;

( and then passing the ion beams through an ion deoelerator; and,decreasing the ion velocity to thermal velocities of the order of a fewelectron volts and,

(h) finally impinging and discharging said heavier ion beam on saidsilicon semiconductor body to form an adherent layer thereon whilesimultaneously passing the separated lighter ions to a collector.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS10/1958 Germany.

1/1957 Great Britain.

OTHER REFERENCES Powell et 'al.: Vapor Plating, pp. -46, 1955.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, WINSTON A. DOUGLAS,

Examiners.

G. E. BATTIST, R. MIHAL-EK, Assistant Examiners.

